The present invention relates to a method and arrangement in an inverter for the purpose of reducing the effects related to the temperature variation of semiconductor components.
An inverter is an electric device in which it is possible to generate a voltage with variable frequencies. Inverters are typically used in motors to control them at an altering frequency or when transferring electric power back to the network, in which case the inverter generates a voltage whose frequency corresponds to that of the network. Such an inverter supplying the network is generally called a network inverter.
Today, IGB transistors (Insulated Gate Bipolar Transistor, IGBT), which are fast, gate-controlled components, are typically used as power semiconductors of inventers, i.e. the switch components generating the output voltage. The current-carrying capacity of the largest IGBT components is several hundreds of amperes and their maximum voltage is thousands of volts. In inverters, switch components are used purely as switches, in which case they have two states, i.e. fully conducting and fully blocking, in practice. Changing states is done as fast as possible to avoid a concurrent voltage and current in the component.
The IGBT mentioned herein is a component made up of several parts and, at the same time, of several pieces having different thermal resistances. The semiconductor components can be considered to be formed of a baseboard, substrate, and the actual semiconductor elements, i.e. chips. The function of the baseboard is to conduct the heat generated in the component to cooling fins or the like. The substrate is on top of the baseboard and the chip is fastened thereto. It is clear that the chip as a resistive component heats up the fastest and the most due to the current running through the component. The baseboard, in turn, heats up the slowest and the least of the component parts, i.e. it has the highest temperature time constant due to cooling in part and in part to the distribution of heat to the large volume of the baseboard.
The different parts of the semiconductor components not only have different temperature time constants, but also different thermal expansion coefficients. The thermal expansion coefficient indicates the size of the expansion caused by temperature change in a piece. Because the parts of a semiconductor component are tightly together, often soldered, mechanical forces occur between them due to expansions of different sizes and strain the component and before long destroy it.
This problem of heat stress is especially big when power semiconductors are loaded in cycles. Cyclic load refers to a load that is not even, but is formed by situations in which the power semiconductor's load is high for a time and low thereafter. Such a load generates a lot of temperature variation in the power semiconductor as the temperature rises strongly during high load, i.e. current, and goes down as the load is reduced. Cyclic load ages a power semiconductor prematurely.
Examples of cyclic drives in an inverter are crane, centrifuge, and lift drives. For instance in centrifuge drives, the inverter controls the motor to rotate the centrifuge that requires a high torque to accelerate, which means a high current and a high temperature increase in the semiconductor. After acceleration, the centrifuge is rotated at operating speed, in which case the output current of the inverter decreases significantly as the required torque decreases. The semiconductor component that heated up during the acceleration now starts to cool. If the centrifuge is further slowed regeneratively, i.e. by using the motor as a generator, a high current again passes through the switch components and the components heat up. The same applies to lift and crane drives and other cyclic drives.
One current method of dimensioning drives is to do it on the basis of the semiconductor temperature variation, i.e. amplitude, caused by the cyclic load. Semiconductor manufacturers indicate the probable number of cycles endured by the semiconductor as a function of temperature variation. As the temperature variation decreases, the allowed maximum number of cycles increases.
Today, inverters, like other electric devices, are cooled actively by fans or liquid cooling. In such solutions, heat is transferred away from the device to cool the components of the device that heat up. Generally, this is done by constant cooling, in which case, despite the temperature of the power semiconductors, the cooling fan or pump operates at a constant rotation rate. In some solutions, it is also known to use cooling that changes directly according to temperature, in which case as the production of heat increases, cooling power is increased to restrict increase in temperature.
It is clear that by using a fan to cool the components, the maximum temperature of the components can be brought down. The use of a fan only does not, however, make it possible to decrease the temperature profile of the component so as to avoid the problems caused by the temperature changes especially in cyclic drives.